This invention relates generally to semiconductor manufacturing systems, more particularly to cleaning apparatus forming a part of semiconductor manufacturing systems, and most particularly to high-purity cleaning systems for flat workpieces.
The production of integrated circuits requires very clean systems and processes. This is because integrated circuits and other semiconductor devices often have line-widths in the sub-micron range. Even very tiny particles adhering to the surface of a semiconductor wafer during the integrated circuit manufacturing process have the potential of destroying the functionality of integrated circuits on the wafer. The semiconductor manufacturing industry therefore goes to great lengths to keep the semiconductor manufacturing equipment, and the surrounding environment, in a very clean condition.
Semiconductor manufacturing is typically accomplished within “clean rooms” which are categorized by “Class.” For example, a Class 100 clean room will have no more than 100 airborne particles of a certain size per cubic foot. A Class 10 clean room will have only 10 such particles per cubic foot, a Class 1 clean room will have only 1 particle per cubic foot, etc. In modern semiconductor manufacturing plants, Class 1 clean rooms are often used.
Integrated circuits are typically formed in multiple copies on a single semiconductor wafer. This semiconductor wafer used most frequently is made from a very pure silicon material, and are typically 6–12 inches in diameter. These wafers, as received from the manufacturer, are very clean. They may, for example, have fewer than a dozen particles of a given sub-micron size per wafer. Ultimately, these wafers are processed by a series of deposition, masking, etching, implantation, etc. operations in order to form a number of integrated circuits on a surface. The wafer is then cut into “die”, each of which includes a single integrated circuit chip. Operative die are packaged and sold as the final integrated circuits.
The “yield” from a semiconductor wafer is defined as the total number of operative die divided by the total number of die on the wafer. The cost-per-integrated circuit is directly related to the yield of the wafer. Since a leading cause of inoperative die are particles that were present on the surface of the wafer during the manufacturing process, it is imperative to keep the wafer surface as clean as possible during the manufacturing process.
Unfortunately, many of the processes to which a wafer is exposed during the manufacturing process are inherently dirty. For example, an etching process can create a large number of particles on the surface of a wafer. As another example, chemi-mechanical polishing (CMP), which is increasingly used to replace isotropic etching processes, uses chemicals and fine particles to grind the surface of a wafer. Chemi-mechanical polishing is also sometimes referred to as “chemi-mechanical planarization”, since it is often used for planarization purposes. It will therefore be appreciated that CMP processes generate a great many particles on the surface of the wafer which must be removed to maintain a reasonable yield from the wafer.
The prior art teaches a number of methods for cleaning wafers. The most common is an aqueous or chemical bath into which the wafer is dipped. However, these “wet” or “dip” processes have a number of deficiencies. For one, wet processes are generally incompatible with cluster tool processing. Wet cleaning processes have traditionally been batch processes where a group or batch (usually 25) wafers was cleaned, rinsed, and dried together in a cassette. Since cluster tools process wafers singly (in series), wafers from cluster tools must be loaded into cassettes and then transported to cleaning areas by human operators. This requires that the wafers be loaded into plastic cassettes that are placed into dip tanks for chemical processing. These cassettes are then placed into spin/rinse dryers for final rinsing and drying. This process consumes time, costs money, and exposes the wafers to the possibility of contamination during transport. As wafer sizes get larger, the transportation and handling of wafers in cassette batches becomes much more difficult and less practical.
To aid in the removal of particles, traditional wet cleaning equipment has used ultrasonic (or Megasonic) high frequency agitation, rotating brushes, or high velocity liquid jets. High frequency agitation has been proven effective at removing particles, but the difficulty lies in preventing redeposit of particles from the solution once the ultrasound energy stops. Since the wafer must often be removed from the solution through a gas liquid interface, it must be removed through a zone where particles may concentrate, recontaminating the wafer upon exit. To avoid this, it has been proposed to provide continuous liquid flow at the interface, but high overflow rates are required to keep particle counts low and this may result in excessive water or chemical consumption.
Rotating brushes have also been used, where the bristles have been chemically treated to modify solution Zeta potential, therefore attracting particles from the wafer surface to the brush. However, for these to work, the nature of the particles must be such that they are attracted to the charge on the brushes. In addition, cleaning the brushes may be critical since dirty brushes (having accumulated a lot of particles) will eventually recontaminate the wafer surfaces, reducing cleaning effectiveness. Liquid jets must be very high velocity in order to result in a fluid boundary layer on the order of the particle sizes (well below 0.5 micron). These high velocities can damage surfaces due to erosion, especially with patterned substrates with various surface topography.
Wet processes do not tend to be very effective at removing all particles, and will actually add particles to the surface of the wafer when the cleaning solution becomes dirty. In addition, the aqueous or liquid phase contains particles that are about 3 orders of magnitude higher (per cubic meter) than those found in the gas phase. This is in part due to the fact that filtration technology is about two to three orders of magnitude less effective for the liquid phase than for the gas phase. Further, even the purest of water has the propensity to grow contaminating microorganisms. Because of these factors, there has been a dedicated focus for many years on “dry” processing utilizing gas phase processes.
The number of particles added to a wafer by the solution is dependent on the concentration equilibrium between the particles in the solution immediately in contact with the wafer and those on the surface. Soaps and surfactants will effectively reduce the “apparent” particle concentration by tying or solvating the particles to organic components in solution. Soaps are not widely used in the semiconductor industry for cleaning of high purity Si wafers because these same surfactants will also contaminate the wafer surfaces. In the case of very dirty wafers and very clean solutions, there will be a tendency for particles to move into solution (assuming charge effects and solution chemistry permits this). In the case of clean wafers and dirty solutions, the opposite can occur. The ultimate baseline test of a cleaning system is to measure particles added to or removed from “virgin prime” substrates which have very few particles on their surface. The better the cleaning system, the fewer the particle adders will be seen.
A typical commercial wafer cleaning apparatus (such as a spin rinse dryer) will always add particles to prime substrates, even when using ultra pure deionized water. This is because, no matter how pure the water source, there is always present particle and bacterial contamination. Only the very sophisticated and highly proprietary final cleaning processes used by the original equipment manufacturers (OEMs) of the silicon substrates, i.e. the wafer manufacturers themselves, can actually remove particles from prime quality wafers. These processes are too complex and expensive to be used in the production fabs.
For the foregoing reasons, wet wafer process cleaning has always been deemed by production fabs as a necessary evil, and for many years effort was focused on developing so called dry cleaning processes which used reactive gasses and plasmas to try and remove particle contaminates. With the advent of CMP, the practicality of using dry processes to remove large levels of contamination has been considerably reduced.
Recently, an improved cleaning process apparatus has been introduced by Verteq, Inc. This process is the subject of a number of recent U.S. patents: U.S. Pat. No. 6,463,938, U.S. Pat. No. 6,295,999, U.S. Pat. No. 6,140,744, and U.S. Pat. No. 6,039,059. This apparatus, marketed under the name “Goldfinger®”, uses a quartz rod coupled to a vibrational transducer to introduce megasonic sound energy into a shallow fluid layer on the surface of a rotating silicon substrate. This machine has been shown to be useful in removing the large particle concentrations present on CMP processed wafers. However, one of the weaknesses of this apparatus is low throughput. Since the machine is designed to handle a single wafer at a time, the process time per wafer must be short in able to produce enough wafers per hour to keep up with other process equipment. Typically, it is desirable to achieve throughputs of over 60 wafers per hour, or 60 seconds per wafer process time. This time must include cleaning, rinsing, and drying time. Since the manufacturer uses liquid jets to introduce fluid onto the surface of the wafer, drying becomes one of the limiting steps in achieving the throughput goal. Drying is accomplished by spinning the horizontally supported substrate after retracting the quartz rod. Applying heated liquid can speed the process, but there is a limit to how much improvement in drying time is obtained because hot liquids introduce condensable vapors into the clean room environment, which are not desirable. Additionally, the larger diameter wafers loose quite a bit of heat due to convective heat losses, and the liquids quickly cool once on the surface, negating the effect of heating them in the first place. The rinsing and drying process utilized by the Goldfinger® machine also suffers from some of the other purity issues mentioned above, in that the liquid water or an aqueous chemical used in rinsing will contain some level of particle contamination.
It would be desirable to obtain the benefits of the surface megasonic cleaning process without the throughput or purity limitations of the prior art.
The use of high purity steam has some potential advantages when compared to conventional wet cleaning. If done correctly, high purity water can be vaporized into a high purity gas (steam), then condensed directly onto the wafer surface. UHP steam is potentially devoid of any ions and contaminates, including bacteria and particles, and will be a very aggressive solvent for surface contaminants. However, it is very important that adequate amounts of steam are applied to the wafer surface, and that the contaminants are flushed uniformly from the surface. Condensing steam at 1 atmosphere pressure will also raise the wafer temperature to nearly 100° C., making residue free drying a possibility.
The prior art has taught the possibility of using steam to clean silicon wafers for a number of years. For example, patents U.S. Pat. No. 4,186,032 and U.S. Pat. No. 4,079,522, mention the use of an inclined heat sink to process wafers one at a time by condensing steam on the wafer surface and allowing the condensate to drain off by gravity. However, this method will not produce uniformly clean wafers due to the fixed orientation of the wafer being cleaned, resulting in potential particle gradients bottom to top. Further, the process as described is quite lengthy, requiring many minutes per wafer. In addition, the technique has no provision for backside cleaning (i.e. the cleaning of the surface of the wafer opposite from the active surface of the wafer) that would not recontaminate the front surface. There is also no mention of purifying the steam prior to condensation, which is critical for particle free performance, since steam (like any gas) can contain aerosols and particles that will contaminate the wafer surface upon condensation.
The prior art discloses the use of steam or condensing water vapor to clean silicon wafers. Typically, such prior art discloses the contact of a wafer cassette or single wafer with saturated steam. The prior art therefore typically ignores the requirement to provide adequate “heat sinking” behind the wafer to condense sufficient steam to clean. Without the heat sink, only enough steam will condense to raise the temperature of the wafer from its input temperature to 100° C., e.g. only a few cubic cm. To condense sufficient quantities of water for adequate cleaning, on the order of a 100–200 cc/min of condensate, a heat sinking of many kilowatts is required. This is perhaps the reason condensing steam has not been utilized for conventional batch cassette processing. It is not easy or practical to heat sink each wafer adequately in a cassette.
It would therefore be desirable to have a cleaning method and apparatus which is both more effective than cleaning methods and apparatus of the prior art and which can become an integral part of a semiconductor manufacturing system.